This invention relates generally to a process for electrolytically producing an alkali metal carbonate. More particularly it relates to an improved process for electrolytically producing an alkali metal carbonate directly in a membrane cell employing particular permselective cation-exchange membranes and operating conditions.
It is known that alkali metal carbonate can be electrolytically produced directly from alkali metal chlorides in diaphragm and membrane cells by introducing carbon dioxide into the catholyte. However, processes presently known have deficiencies in one or more important particulars.
For example, U.S. Pat. No. 3,374,164 discloses that modern diaphragm cells, wherein the diaphragm is affixed to the cathode, while capable of operating at 95 to 95% electrolytic efficiency, only convert 60% of the alkali metal ions migrating through the diaphragm to the carbonate salt. Further, the patent discloses that even when the diaphragm is separated from the cathode and carbon dioxide is introduced into the resulting space, conversion efficiency can only be raised to 80% maximum. In either case, the carbonate salt is contaminated with unacceptable concentrations of chloride salt that must be removed by additional separate purification steps raising costs to noncompetitive levels.
U.S. Pat. No. 2,967,807, on the other hand, in Example III discloses that membrane cells of the prior art also produce carbonate salts having an appreciable level of chloride salt impurities. Additionally, the operating conditions specified in this Example, viz. 90 amperes/ft..sup.2 (0.62 amperes/in..sup.2) at an imposed voltage of 3.8-4.2, indicate that membrane cells require appreciably more energy and, thus, are considerably less efficient than diaphragm or mercury cells, and that they, therefore, would be unsuitable economically for the commercial production of alkali metal carbonate salts.
Because of these deficiencies, a significant quantity of high purity alkali metal carbonates, and especially potassium carbonate, is commercially made by carbonating alkali metal hydroxides produced from mercury cells. This, of course, involves the installation of auxiliary carbonation equipment and separate additional processing steps, both of which increase costs. However, the factor most militating against the use of mercury cells for the production of alkali metal carbonates is their potential to contaminate the environment. To minimize such contamination to acceptable levels, considerable monies must be spent for pollution control means and significantly higher operating costs are entailed.
In view of the foregoing, the industry has endeavored to develop processes that are capable of producing alkali metal carbonates having the purity of mercury cell products and at the same time the nonpolluting characteristics of the diaphragm and membrane cell processes. To date, this has not been achieved.